The present invention relates to a closed cell foamed polyolefin film or sheet product and in particular to such products suitable for use in the production of wall coverings, which term also includes ceiling coverings.
When intended for use as a wall covering base film, it is desirable that such films have a low bulk density, so that they are light and easy to handle. Such films must also be readily printable and have a uniform smooth surface finish. These films should also be abrasion resistant.
It has been proposed, in British Patent Specification No. 1,220,053, to make wall coverings using an open cell foamed film formed from low density polyethylene. To overcome certain disadvantages, notably abrasion resistance, of such wall coverings, it has been proposed in British Patent Specification No. 1,306,373 to employ a mixture of high density and low density polyethylene in specified proportions.
When using a mixture of high and low density polyethylene, the extrusion temperature required to obtain satisfactory extrusion in higher than that required with low density polyethylene alone, because of the higher melting point of high density polyethylene. When using volatile liquid or gaseous blowing agents, such higher temperatures result in the production of open cell foams, and also a relatively large amount of blowing agent is required to obtain a foam of low bulk density.
Open cell foams generally have a relatively poor surface finish compared to closed cell foams. However, it has not been found possible to make closed cell foam films using volatile liquid or gaseous blowing agents from polyolefin mixtures containing more than 10% by weight of high density polyethylene.
It has been proposed, in British Patent Specification No. 1,376,151, to make an open cell foam material from a mixture of low density polyethylene and an ethylene copolymer containing polar groups. Comparative examples in that specification describe closed cell foams made from such mixtures, but no utility therefore is indicated.